(1) Field of the Invention
The present invention relates to the inspection of the interior of containers and particularly to the in-service inspection of nuclear reactor pressure vessels. More specifically, this invention is directed to remotely controllable vessel inspection apparatus and especially to sensor supporting booms characterized by multiple degrees of freedom. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
The core of a pressurized water nuclear reactor, of the type commonly employed by electrical utilities, is housed within a metal container which is referred to as the reactor pressure vessel. The pressure vessel will be generally of cylindrical shape and will, at the bottom thereof, have a hemispherical-shaped end or "bottom head" portion. Additionally, a "top flange" will be welded at the upper end of the vessel cylindrical or "shell" portion and inlet and outlet nozzles will extend through the vessel wall. Because of its dimensions and the gauge of the metal from which it is fabricated, a reactor pressure vessel is formed as segments which are welded together. The pressure vessel is itself housed within a concrete containment structure.
Safety regulations dictate that a reactor pressure vessel, and particularly the weld areas thereof, be inspected prior to use and periodically after the reactor is placed in service. The in-service inspections will be performed in an underwater, radioactive environment and thus require remotely controllable inspection apparatus. The vessel inspections include the non-destructive testing of the vessel wall employing ultrasonic energy. In order to accomplish the testing, a plurality of ultrasound transducers will be mounted on a "sled" which is caused to move along the vessel interior wall. The transducers will typically be spaced a short distances from the vessel wall and the ultrasonic energy coupled via the water in the vessel into the metal. The examination will produce a profile of the cross-section of the vessel wall. The information obtained from this volumetric examination of the metal will, in the case of a pre-service examination, show any inherent defects, particularly in the material around the welds. The in-service examinations will reveal defects which have arisen during use such as stress cracks and intergranular stress corrosion.
It is obviously highly desirable that the measurements taken during a reactor vessel in-service inspection be repeatable. Accordingly, the positioning and movements of the "sled" must be accurately controlled, from a remote location, and must be referenced to a common point or line. It has, in the past, been exceedingly difficult to achieve the desired repeatability of "sled" positioning and movements. Further, and perhaps more significantly, prior in-service reactor pressure vessel inspection apparatus was characterized by inflexibility. That is, for each portion of the vessel interior to be inspected, it was necessary to actually effect mechanical changes to the "sled" supporting arm or the "sled" itself. The necessity of changing the arm or the equipment mounted thereon greatly increased the cost of performing a pressure vessel inspection.
For a general discussion of reactor pressure vessel inspection, reference may be had to U.S. Pat. No. 4,302,286. The inspection apparatus, for example as depicted in FIG. 2 of U.S. Pat. No. 4,302,286, is exemplary of the prior art wherein the ultrasound transducer supporting arm is capable of movement only about a single pivot axis.